A cold, slushy ocean lying deep beneath Pluto’s bright, heart-shaped Tombaugh Regio is the best explanation for features revealed by NASA’s New Horizons probe, according to new research.
This cutaway image of Pluto shows a section through the area of Sputnik Planitia, with dark blue representing a subsurface ocean and light blue for the frozen crust. Image credit: Pam Engebretson.
For several decades, researchers have observed that Tombaugh Regio aligns almost exactly opposite from Charon, Pluto’s largest moon, in a locked orientation that has lacked a convincing explanation.
An underground ocean, the new data suggest, may have served as a ‘gravitational anomaly,’ or weight, which would factor heavily in Pluto and Charon’s gravitational tug-of-war.
Over millions of years, Pluto would have spun around, aligning its ocean and the heart-shaped region above it, almost exactly opposite along the line connecting the dwarf planet and Charon.
“People had considered whether you could get a subsurface layer of water somewhere on Pluto,” said New Horizons co-investigator Richard Binzel, a professor in the Department of Earth, Atmospheric and Planetary Sciences at MIT and co-author of a paper on the new findings published this week in the journal Nature.
“What’s surprising is that we would have any information from a flyby that would give a compelling argument as to why there might be a subsurface ocean there. Pluto just continues to surprise us.”
F. Nimmo et al found evidence of a cold, slushy beneath Pluto’s heart-shaped region. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute.
During its flyby of Pluto in July 2015, New Horizons collected measurements of surface features, including the dimensions of Pluto’s heart-shaped region.
In particular, the probe focused on a circular region in its left ‘ventricle,’ named Sputnik Planitia, which is a topographic depression approximately 621 miles (1,000 km) in diameter possibly representing an ancient impact basin.
“The interior of Sputnik Planitia is characterized by a smooth, craterless plain 1.9-2.5 miles (3-4 km) beneath the surrounding rugged uplands, and represents the surface of a massive unit of actively convecting volatile ices several miles thick,” the authors said.
From the New Horizons data, the researchers determined the size and depth of Sputnik Planitia.
They also found that Tombaugh Regio, and Sputnik Planitia in particular, is aligned almost exactly opposite from Charon.
The likelihood that this is just a coincidence is only 5%, so the alignment suggests that extra mass in that location interacted with tidal forces between Pluto and Charon to reorient the dwarf planet.
“It’s a big, elliptical hole in the ground, so the extra weight must be hiding somewhere beneath the surface. And an ocean is a natural way to get that,” added first author Francis Nimmo, a professor in the Department of Earth and Planetary Sciences at the University of California, Santa Cruz.
Another paper in the same issue of Nature, led by University of Arizona researcher Dr. James Keane, also argues for reorientation and points to fractures on Pluto as evidence that this happened.
“The New Horizons data say it’s not only opposite Charon, but it’s really close to being almost exactly opposite,” Prof. Binzel said.
“So we asked, what’s the chance of that randomly happening? And it’s less than 5% that it would be so perfectly opposite. And then the question becomes, what was it that caused this alignment?”
According to the researchers, Sputnik Planitia was most likely created by the impact of a giant meteorite, which would have blasted away a huge amount of Pluto’s icy crust.
With a subsurface ocean, the response to this would be an upwelling of water pushing up against the thinned and weakened crust of ice.
At equilibrium, because water is denser than ice, that would still leave a fairly deep basin with a thin crust of ice over the upwelled mass of water.
“At that point, there is no extra mass at Sputnik Planitia. What happens then is the ice shell gets cold and strong, and the basin fills with nitrogen ice. That nitrogen represents the excess mass,” Prof. Nimmo said.
The team also considered whether the extra mass could be provided by just a deep crater filled with nitrogen ice, with no upwelling of a subsurface ocean. But their calculations showed that this would require an implausibly deep layer of nitrogen, more than 25 miles (40 km) thick.
The authors found that a nitrogen layer about 4 miles (7 km) thick above a subsurface ocean provides enough mass to create a ‘positive gravity anomaly’ consistent with the observations.
“We tried to think of other ways to get a positive gravity anomaly, and none of them look as likely as a subsurface ocean,” Prof. Nimmo said.
The scientists came up with the reorientation hypothesis, and developed the subsurface ocean scenario, which is analogous to what occurred on the Moon, where positive gravity anomalies have been accurately measured for several large impact basins.
Instead of a subsurface ocean, however, the dense mantle material beneath the lunar crust pushed up against the thinned crust of the impact basins. Lava flows then flooded the basins, adding the extra mass. On icy Pluto, the basin filled with frozen nitrogen.
“There’s plenty of nitrogen in Pluto’s atmosphere, and either it preferentially freezes out in this low basin, or it freezes out in the high areas surrounding the basin and flows down as glaciers,” Prof. Nimmo said.
New Horizons images show what appear to be nitrogen glaciers flowing out of mountainous terrain around Sputnik Planitia.
“As for the subsurface ocean, we suspects it is mostly water with some kind of antifreeze in it, probably ammonia,” Prof. Nimmo said.
“The slow refreezing of the ocean would put stress on the icy shell, causing fractures consistent with features seen in the New Horizons images.”
“There are other large objects in the Kuiper belt that are similar to Pluto in size and density, and they probably also have subsurface oceans,” he said.
“When we look at these other objects, they may be equally interesting, not just frozen snowballs.”
_____
F. Nimmo et al. Reorientation of Sputnik Planitia implies a subsurface ocean on Pluto. Nature, published online November 16, 2016; doi: 10.1038/nature20148
J.T. Keane et al. Reorientation and faulting of Pluto due to volatile loading within Sputnik Planitia. Nature, published online November 16, 2016; doi: 10.1038/nature20120
